There are many active studies on mechanisms and therapeutic agents for various immune disorders or immune depression caused by carcinostatic drugs. It is known that thymulin, a nonapeptide produced in the thymus, forms a complex with zinc and improves a depressed immunity (Med. Oncol. & Tumor Pharmacother. 6, 25-29, 1989).
However, there remain many problems in practical use of thymulin. For example, the availability of thymulin is limited due to a small yield in the thymus and duration of the activity thereof is short because thymulin is easily decomposed by endogenous enzymes.
Therefore, synthetic compounds are desired which have a long lasting activity and can be prepared in a large amount. As such synthetic compounds, lactone or lactam compounds having two intramolecular sulfur atoms in the side chains are proposed (PCT/JP92/00002). As shown in the PCT application, the lactone or lactam compounds show excellent thymulin-like activities.
However, it is further desired to study other useful compounds such as different heterocycles.
The inventors focused on lactone derivatives having two sulfur atoms in their side chains, which were disclosed in PCT/JP92/00002, and studied to take one sulfur atom in a ring system, and succeeded in obtaining novel tetrahydrothiophene derivatives which have excellent thymulin-like activities.